perl593delta.pod

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=head1 NAME

perl593delta - what is new for perl v5.9.3

=head1 DESCRIPTION

This document describes differences between the 5.9.2 and the 5.9.3
development releases. See L<perl590delta>, L<perl591delta> and
L<perl592delta> for the differences between 5.8.0 and 5.9.2.

=head1 Incompatible Changes

=head2 Parsing of C<-f _>

The identifier C<_> is now forced to be a bareword after a filetest
operator. This solves a number of misparsing issues when a global C<_>
subroutine is defined.

=head2 C<mkdir()>

C<mkdir()> without arguments now defaults to C<$_>.

=head2 Magic goto and eval

The construct C<eval { goto &foo }> is now disallowed. (Note that the
similar construct, but with C<eval("")> instead, was already forbidden.)

=head2 C<$#> has been removed

The deprecated C<$#> variable (output format for numbers) has been
removed. A new warning, C<$# is no longer supported>, has been added.

=head2 C<:unique>

The C<:unique> attribute has been made a no-op, since its current
implementation was fundamentally flawed and not threadsafe.

=head2 Scoping of the C<sort> pragma

The C<sort> pragma is now lexically scoped. Its effect used to be global.

=head1 Core Enhancements

=head2 The C<feature> pragma

The C<feature> pragma is used to enable new syntax that would break Perl's
backwards-compatibility with older releases of the language. It's a lexical
pragma, like C<strict> or C<warnings>.

Currently the following new features are available: C<switch> (adds a
switch statement), C<~~> (adds a Perl 6-like smart match operator), C<say>
(adds a C<say> built-in function), and C<err> (adds an C<err> keyword).
Those features are described below.

Note that C<err> low-precedence defined-or operator used to be enabled by
default (although as a weak keyword, meaning that any function would
override it). It's now only recognized when explicitly turned on (and is
then a regular keyword).

Those features, and the C<feature> pragma itself, have been contributed by
Robin Houston.

=head2 Switch and Smart Match operator

Perl 5 now has a switch statement. It's available when C<use feature
'switch'> is in effect. This feature introduces three new keywords,
C<given>, C<when>, and C<default>:

    given ($foo) {
	when (/^abc/) { $abc = 1; }
	when (/^def/) { $def = 1; }
	when (/^xyz/) { $xyz = 1; }
	default { $nothing = 1; }
    }

A more complete description of how Perl matches the switch variable
against the C<when> conditions is given in L<perlsyn/"Switch statements">.

This kind of match is called I<smart match>, and it's also possible to use
it outside of switch statements, via the new C<~~> operator (enabled via
the C<use feature '~~'> directive). See L<perlsyn/"Smart matching in
detail">.

=head2 C<say()>

say() is a new built-in, only available when C<use feature 'say'> is in
effect, that is similar to print(), but that implicitly appends a newline
to the printed string. See L<perlfunc/say>.

=head2 C<CLONE_SKIP()>

Perl has now support for the C<CLONE_SKIP> special subroutine. Like
C<CLONE>, C<CLONE_SKIP> is called once per package; however, it is called
just before cloning starts, and in the context of the parent thread. If it
returns a true value, then no objects of that class will be cloned. See
L<perlmod> for details. (Contributed by Dave Mitchell.)

=head2 C<${^CHILD_ERROR_NATIVE}>

A new internal variable, C<${^CHILD_ERROR_NATIVE}>, gives the native
status returned by the last pipe close, backtick command, successful call
to wait() or waitpid(), or from the system() operator. See L<perlrun> for
details. (Contributed by Gisle Aas.)

=head2 Assertions

The support for assertions, introduced in perl 5.9.0, has been improved.
The syntax for the C<-A> command-line switch has changed; it now accepts
an optional module name, defaulting to C<assertions::activate>. See
L<assertions> and L<perlrun>. (Contributed by Salvador Fandi駉 Garc韆.)

=head2 Unicode Character Database 4.1.0

The copy of the Unicode Character Database included in Perl 5.9 has
been updated to 4.1.0.

=head2 C<no VERSION>

You can now use C<no> followed by a version number to specify that you
want to use a version of perl older than the specified one.

=head2 Recursive sort subs

You can now use recursive subroutines with sort(), thanks to Robin Houston.

=head2 Effect of pragmas in eval

The compile-time value of the C<%^H> hint variable can now propagate into
eval("")uated code. This makes it more useful to implement lexical
pragmas.

As a side-effect of this, the overloaded-ness of constants now propagates
into eval("").

=head2 New B<-E> command-line switch

B<-E> is equivalent to B<-e>, but it implicitly enables all
optional features (like C<use feature ":5.10">).

=head2 C<chdir>, C<chmod> and C<chown> on filehandles

C<chdir>, C<chmod> and C<chown> can now work on filehandles as well as
filenames, if the system supports respectively C<fchdir>, C<fchmod> and
C<fchown>, thanks to a patch provided by Gisle Aas.

=head2 OS groups

C<$(> and C<$)> now return groups in the order where the OS returns them,
thanks to Gisle Aas. This wasn't previously the case.

=head1 Modules and Pragmata

=head2 New Core Modules

=over 4

=item *

A new pragma, C<feature>, has been added; see above in L</"Core
Enhancements">.

=item *

C<assertions::compat>, also available on CPAN, allows the use of assertions on
perl versions prior to 5.9.0 (that is the first one to natively support
them).

=item *

C<Math::BigInt::FastCalc> is an XS-enabled, and thus faster, version of
C<Math::BigInt::Calc>.

=item *

C<Compress::Zlib> is an interface to the zlib compression library. It
comes with a bundled version of zlib, so having a working zlib is not a
prerequisite to install it. It's used by C<Archive::Tar> (see below).

=item *

C<IO::Zlib> is an C<IO::>-style interface to C<Compress::Zlib>.

=item *

C<Archive::Tar> is a module to manipulate C<tar> archives.

=item *

C<Digest::SHA> is a module used to calculate many types of SHA digests,
has been included for SHA support in the CPAN module.

=item *

C<ExtUtils::CBuilder> and C<ExtUtils::ParseXS> have been added.

=back

=head1 Utility Changes

=head2 C<ptar>

C<ptar> is a pure perl implementation of C<tar>, that comes with
C<Archive::Tar>.

=head2 C<ptardiff>

C<ptardiff> is a small script used to generate a diff between the contents
of a tar archive and a directory tree. Like C<ptar>, it comes with
C<Archive::Tar>.

=head2 C<shasum>

This command-line utility, used to print or to check SHA digests, comes
with the new C<Digest::SHA> module.

=head2 C<h2xs> enhancements

C<h2xs> implements a new option C<--use-xsloader> to force use of
C<XSLoader> even in backwards compatible modules.

The handling of authors' names that had apostrophes has been fixed.

Any enums with negative values are now skipped.

=head2 C<perlivp> enhancements

C<perlivp> no longer checks for F<*.ph> files by default.  Use the new C<-a>
option to run I<all> tests.

=head1 Documentation

=head2 Perl Glossary

The L<perlglossary> manpage is a glossary of terms used in the Perl
documentation, technical and otherwise, kindly provided by O'Reilly Media,
Inc.

L<perltodo> now lists a rough roadmap to Perl 5.10.

=head1 Performance Enhancements

=head2 XS-assisted SWASHGET

Some pure-perl code that perl was using to retrieve Unicode properties and
transliteration mappings has been reimplemented in XS.

=head2 Constant subroutines

The interpreter internals now support a far more memory efficient form of
inlineable constants. Storing a reference to a constant value in a symbol
table is equivalent to a full typeglob referencing a constant subroutine,
but using about 400 bytes less memory. This proxy constant subroutine is
automatically upgraded to a real typeglob with subroutine if necessary.
The approach taken is analogous to the existing space optimisation for
subroutine stub declarations, which are stored as plain scalars in place
of the full typeglob.

Several of the core modules have been converted to use this feature for
their system dependent constants - as a result C<use POSIX;> now takes about
200K less memory.

=head2 C<PERL_DONT_CREATE_GVSV>

The new compilation flag C<PERL_DONT_CREATE_GVSV>, introduced as an option
in perl 5.8.8, is turned on by default in perl 5.9.3. It prevents perl
from creating an empty scalar with every new typeglob. See L<perl588delta>
for details.

=head2 Weak references are cheaper

Weak reference creation is now I<O(1)> rather than I<O(n)>, courtesy of
Nicholas Clark. Weak reference deletion remains I<O(n)>, but if deletion only
happens at program exit, it may be skipped completely.

=head2 sort() enhancements